Leaking Fuel Tanks

by Cruising Editor on 13 Aug 2006 Leaking Fuel Tank? Is yours accessible? How would you remove your fuel tank for repair or replacement if the need arose?

A leak in a fuel tank can be as serious or as simple as your original boat designers decided. If they decided to place the fuel tank in such a position that it cannot be removed without major deconstruction of the boat, then you could end up with a VERY major job.

Recently, this is just what happened to us. The fuel tank, which had been made out of steel, could be accessed from within a deck lazarette, but was too large to be removed through the lazarette. This meant that the only way to remove it was to first remove the engine!

The side effect of this discovery was a significant quantity of smoke coming from the Skipper’s ears, and any thought of repair had to be delayed until after this environmental problem had been dealt with.

After discussion with various ‘experts’, we came up with these five alternative solutions:

1. We could remove it by cutting it up in pieces, thereby destroying the old tank. We could then use the ‘pieced together’ old tank for a mould to make several smaller tanks that could be fitted in via the available lazarette. These tanks could be made of either steel or fibreglass (fibreglass with special resins to counter the effects of diesel fuel in contact). The disadvantage of this method is, apart from it being a major job, we would lose significant fuel storage, as the several tanks could not hold as much as the original

2. We could leave the old tank in place, and buy some ‘off the shelf’ plastic tanks and place them on top of the old tank. Here again we would lose fuel storage space, and we would also lose lazarette space. An advantage would be that it is a quick and easy solution with no unknowns. A disadvantage is that the old tank would still be in place and would have to be dealt with at some future date.

3. We could leave the old tank in place, cut just the top off the tank, and place flexible rubber tanks inside the old tank. The advantage of this was that it was quick and easy, also not very expensive. However, it turned out that the shape of the old tank was a V shape, fitted to the shape of the boat, and the flexible rubber tanks were square. We remain unsure today whether they would be badly affected by placing them inside such a differently shaped container.

4. We could cut the top from the existing fuel tank and use it as a mould to create a fibreglass tank inside it. The disadvantage of this was that as the old tank continued to rust, it might expand and put pressure on the sides of the boat.

5. We could cut the top from the existing fuel tank, not use it as a mould, but make a slightly smaller fibreglass tank by accurately measuring the inside of the tank. The only disadvantage of this was that we would lose some fuel storage, but not a great deal.

No 5 solution is the solution that we chose.


While investigating the alternatives, we came up with lots of interesting information about replacing old fuel tanks, some not valid for our situation, but may be valid for yours. Most of the information came from USA Marine

Suitable materials for the construction of marine fuel tanks and appropriate advice concerning their use:

Aluminium: This is one of the most commonly used materials. You'll have little problem finding either a prefabricated tank or having one custom fabricated locally. The main concern with aluminum is oxidation pitting which will lead eventually to leaking. This can be averted by applying a 16 - 18 mil exterior coating of coal tar epoxy.

Stainless steel (should also be cold tar epoxy coated.)

Polyethylene (Must be a molded construction only - NOT fabricated, and must be high grade fuel rated material)

ISO Polyester fiberglass (Do not cold tar epoxy coat.)
Do not bond your new tank into the boat. To achieve a form fit, first wrap the tank tightly with polyethylene sheeting, (Visquene), center the tank where you want it, and cast in the voids with a 2 part, 4 lb density polyurethane foam.

You'll want to support the tank above the hull surface with either strips of PVC foam, (Divinycell for example), or thick rubber. Allow enough space for the foam mix to easily fill in under the tank. An inch of thickness is fine. You'll also need to temporarily lock the tank down so that the foam wont float it up during the expansion phase.

After the foam cures trim it back flush with the top of the tank.

Next, cover this level with the poly sheeting, including the exposed sides and cast in a solid block of foam that can then be lifted out. Trim away the excess to allow the deck plate to be reinstalled and cut in, as necessary, access holes for your fill tube, pickup fitting, and sending unit fitting. This plug can be lightly glassed for longevity if desired.

After all of your foam work is done, remove the tank and discard all of the poly sheeting. You now have a form fitted cavity and will have no problem with removing the tank at some future date if need be, and your tank should meet or exceed the life expectancy of the boat.

If you are having your tank custom built, be sure to use a fabricator that is qualified to build Marine Fuel Tanks. If you are currently powered with inboard gasoline engines, remember that you, (or the next owner), may want to convert to diesel power. While gas power doesn't require fuel return fittings, diesel does, and also they need to be 1/8' larger in diameter. Have these fittings installed now so that the tank will not have to be refitted later... The returns can simply be capped off for gasoline use. The same applies to fittings for a generator.

On the pickup tubes: The engine tubes should drop to a lower level than the generator tube. This will act as a warning system since the generator will run out of fuel before the engines do. All pickup tube ends should be cut at a 45° angle and not extend all the way to the bottom of the tank. DO NOT use any sort of strainer devices inside of the tank. They'll just plug up eventually and you'll have to dismantle the system to clean or replace. Not fun...